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HELP! Server side validation for phone number

Hello, my name is Yuni. I need help in my assignment! Hope any kind soul could help me solve my problem!

My problem is that whenever I type alphabet into the contact number field, there will be error. How could I solve the error by typing what code?
I can validate the minimum input length and client side coding, but I do not know how to code in order to prevent error when typing in invalid answer.

Below is an example of the error if I type "a" into [contact no] field.